The present invention relates to optical amplifiers, and more particularly, to optical amplifiers such as those used in fiber-optic communications networks.
In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber. Optical amplifiers are used in such networks to amplify optical signals that have been subject to attenuation over fiber-optic links. It is desirable for amplifiers to have wide optical bandwidths, so that a large number of communications channels may be carried.
A typical amplifier may include erbium-doped fiber amplifier components that are pumped with diode lasers. Raman amplifiers have also been used that provide optical gain through stimulated Raman scattering. Erbium-doped amplifiers may be configured to operate in the C-band wavelength region (approximately 1530 nm to 1565 nm) or the L-band wavelength region (approximately 1565 nm to 1615 nm). Erbium amplifiers that handle both C-band and L-band signals have also been developed. Such amplifiers use a wavelength demultiplexer and wavelength multiplexer to route signals through parallel C-band and L-band amplifier branches. Signals in the C-band are routed through the C-band amplifier branch. Signals in the L-band are routed through the L-band branch. This type of parallel arrangement provides a wide amplifier bandwidth, but may require the use of a guard band to avoid multipath interference problems.
The wavelength and bandwidth of a Raman amplifier may be determined by choosing the wavelengths of the Raman pump or pumps used to produce the gain. However, it may be difficult to produce a Raman amplifier that has a bandwidth larger than 100 nm, because this would cause the longest wavelength pump to be located in the short wavelength region of the signal band.
One possible approach for increasing amplifier bandwidth is to combine amplifiers that operate in different wavelength bands in series. With this type of arrangement, signals pass sequentially through each of the amplifiers. Guard bands are not required, but there may be stringent requirements for the separate amplifiers. Each amplifier must amplify within its designed band while allowing signals to pass in other bands for amplification by the other amplifiers. In the ideal situation, each amplifier would produce absolutely no loss or noise in the band of the other amplifiers. This would allow amplifiers to be combined in series in any order without any performance degradation. However, it is not possible to produce an amplifier with such ideal characteristics in practice.
When two amplifiers are combined in series, each amplifier generally produces loss in the gain band of the other amplifier. For example, the first amplifier in the series may contribute a loss in the gain band of the second amplifier in the series. This will increase the noise figure of the amplifier in the gain band of the second amplifier. If the order of the amplifiers is reversed, so that the first amplifier follows the second amplifier, the output power in the gain band of the second amplifier will be reduced. In some situations, one amplifier may produce a small gain in the gain band of the other amplifier, rather than a loss. However, all gain is accompanied by noise. The introduction of a poorly performing (low gain) amplifier either before or after a high performance amplifier will therefore degrade performance in the gain band of the high-performance amplifier.
It is an object of the present invention to provide wide-band amplifiers for use in optical communications systems.
It is also an object of the present invention to provide amplifiers for use in optical communications systems in which amplifier stages corresponding to different wavelength bands are interleaved with each other to improve the overall performance of the amplifier.
These and other objects of the invention are accomplished in accordance with the present invention by providing amplifiers having multiple gain stages. Some of the gain stages provide gain in a first wavelength band and some of the gain stages provide gain in a second wavelength band. The gain stages that provide gain in the first band are interleaved with the gain stages that provide gain in the second band.
The gain stages may be based on rare-earth-doped fiber amplifiers, Raman-pumped fiber amplifiers, or any other suitable optical amplifiers.
If desired, gain stages may be used that include parallel amplifier branches. Each of the parallel branches may handle a separate wavelength range.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.